Towards a fully consistent parameterization of modified gravity

TL;DR

This paper discusses the limitations of existing parameterizations of modified gravity on large cosmological scales and proposes a more comprehensive approach inspired by fluid perturbations to improve model independence.

Contribution

It identifies the shortcomings of current parameterizations in the non-quasistatic regime and suggests extending the Parameterized Post-Friedmann formalism using fluid perturbation techniques.

Findings

Most parameterizations are only valid in the quasistatic regime.

Current models fail to capture full behavior on large scales.

Proposes a fluid perturbation approach to improve parameterization.

Abstract

There is a distinct possibility that current and future cosmological data can be used to constrain Einstein's theory of gravity on the very largest scales. To be able to do this in a model-independent way, it makes sense to work with a general parameterization of modified gravity. Such an approach would be analogous to the Parameterized Post-Newtonian (PPN) approach which is used on the scale of the Solar System. A few such parameterizations have been proposed and preliminary constraints have been obtained. We show that the majority of such parameterizations are only exactly applicable in the quasistatic regime. On larger scales they fail to encapsulate the full behaviour of typical models currently under consideration. We suggest that it may be possible to capture the additions to the `Parameterized Post-Friedmann' (PPF) formalism by treating them akin to fluid perturbations.